The present invention relates to a method and apparatus for quantitative testing and evaluation of body joint stiffness. It is intended for patient evaluation in physical medicine and for objective testing of the efficacy of various therapeutic treatments, such as for example heat and anti-inflammatory pharmaceuticals, to reduce joint stiffness in various disease states such as rheumatoid arthritis, scleroderma, and polymyositis.
Joint stiffness is an important component of rheumatoid arthritis and other connective tissue diseases that, along with pain, largely determine the functional capacity of the patient. Heretofore, joint stiffness was measured in situ by the imposition of continuous sinusoidal motion upon a joint. This method confounds passive and active components of resistance movement. Passive properties include elasticity, viscosity, and friction as components of the resistance to motion in joints. Of these, elasticity is by far and away the most important component (i.e., at least 80%) of the total resistance to movement.
Changes in the passive properties of periarticular structures are of greatest importance to evaluation of rheumatoid arthritis, especially with respect to morning stiffness and prediction of later joint deformity. Active processes, measured concurrently with passive properties of the joint when continuous sinusoidal movement is imposed, largely reflect muscle stretch reflexes. In spastic paralysis, for example, hypertonic stretch reflexes may eclipse the passive properties.
One way to separate passive and active properties in force v. displacement tests of joint movement is to block muscle stretch reflexes with local anesthetics. This procedure is not attractive for routine use. Even in non-spastic subjects, muscle stretch reflexes cannot be non-invasively separated from viscoelastic properties of the joint when the joint is accelerated.
The present invention uses an improved approach based on discrete, rather than continuous, movement wherein strain is measured with the joint stationary at each of several discrete positions within its range of motion. Elastic deformation is avoided as the measurements are taken at a high enough data rate to prevent stress relaxation. This method isolates the dominant passive component (elasticity) of resistance to motion in joints. In this way, objective evaluation of disease response to therapeutic treatments, such as anti-inflammatory agents and heat, may guide treatment regimes to more effectively reduce joint stiffness and relieve pain.